Present and future climate research programs require the accurate measurement of total radiation from precise partial earth area footprints as well as total earth viewing measurements. Earth footprints, smaller than the total earth, are of precise angular measurements and are required to develop angular distribution models of reflected and emitted earth radiation. Present climate sensor research programs have contained mechanically scanning sensors which have a sensing angle less than the entire earth angle. These scanning sensors obtain radiation from smaller footprints, as they are scanned on the earth. The mechanical scanning of the sensors has introduced complexities in obtaining, calibrating, and analyzing the radiation measurements.
One problem is that the mechanical sensors require torquers, encoders, and other mechanical component which have a limited life. One mechanically scanned sensor failed after less than twenty months of operation. Future climate research programs will require long-term measurements to b made up to one solar cycle of eleven years. To meet these needs, it is necessary to build a sensor which will operate for up to eleven years, and this obviates the employment of mechanical scanning parts.
When a single scanner is employed on an orbiting satellite, it is normally mechanically scanned in the cross-track direction. The data which is produced is in a direction determined by the spacecraft orbital inclination and not the optimum direction for determining equator-to-pole variations and other angular distributions of interest. The field-of-view pattern on the earth generated by mechanical scanning of the sensor is dictated by the earth curvature and view angle. It would be more desirable to select the field-of-view pattern in each direction which best fits the angular radiation portions of the mathematical models. This can be better accomplished by means of a mosaic array of sensor footprints of the selected field-of-view pattern.
Another problem which arises with the mechanical scanning of a single sensor is that the field-of-view of the scanner must be small if it is to provide reasonable spatial resolution towards the edge of the earth, at low earth angles. This high resolution means a small field-of-view and a high sensitivity requirement. The spatial resolution at nadir is higher than necessary, and the field-of-view at nadir is smaller than necessary for nadir measurements. The high spatial resolution towards the edge of the earth requires a sensor which is of greater sensitivity than the presently available cavity type detectors. Accordingly, for these needs, non-cavity type detectors with optical telescopes are required to produce sufficient gain to give the required instrument precision. Such detectors and optical telescopes have spectral characteristics that prevent them from having a flat response to radiation from the ultraviolet to the far infrared. With the filtering effect of such non-cavity detectors, the spectral responses are difficult to measure and impossible to eliminate completely from the data. Such telescopes also introduce undesirable polarization effects which cannot be eliminated from the resultant measurements. A cavity-type detector with sufficient sensitivity to produce the required precision when viewing a high resolution footprint at or on the earth is required.